Known eye gaze detection systems are disclosed in:
"Human-Computer Interaction Using Eye-gaze input", T.E. Hutchison et al, IEEE Trans. Sys., Man, Cybern., vol. 19, p.1527, 1989;
"Improved video-based eye-gaze detection method", Y. Ebisawa, Proc. IMTC '94 pp. 963-966, 1994;
K.P. White Jr., T.E. Hutchison, and J.M. Carley, Spatially dynamic calibration of an eye-tacking system, IEEE Trans. Syst., Man, Cybern., vol.23 pp. 1162-68, 1993;
A. Gee and R. Cipollo, Non-intrusive gaze tracking for human-computer interaction, Proc. Mech. Mach. Vis. 1994.
Two reflections from the eye are detected, The first is caused by the light passing through the pupil of the eye and being reflected back from the retina. The second, known as "glint" is formed by light reflected from the surface of the cornea. By, for instance, detecting the reflection from the cornea for one source and detecting the glint for several sources of illumination, the position of the eye and its direction of gaze can be calculated.
In such systems, the region in front of a screen which is likely to be occupied by the face of an observer is illuminated, for instance by an infrared light emitting diode. A two dimensional image of the observer face is then captured by a video camera, for instance based on a charge coupled device. The image is then subjected to complex image processing in order to detect the reflection of the light source from the eye.
Arrangements of this type have several disadvantages. For instance, relatively complex image processing is required in order to identify the observer eye. This requires substantial processing capability which in turn requires relatively powerful data processors and/or relatively long processing times. Further, the resolution of detection of the various reflections is limited by the resolution of the video camera. The video camera is required to capture a 2D image of the region of space which may be occupied by the observer so that the reflections which are required to be detected occupy a relatively small portion of the captured image. For video cameras of the charge coupled device (CCD) type, the size of the reflections in the image can become comparable to the intrinsic resolution of the camera. Typically, the whole face of the observer is imaged onto the CCD so that the eyes of the observer are imaged onto a relatively small proportion of the light sensitive surface of the CCD. The CCD has a finite resolution which thus limits the accuracy with which the locations of reflections can be determined. Further, the quality of the light source is an important factor in the accuracy of determination.